The use of sapphire wafers for a lens, such as a cover lens for mobile phones, has advantages over silica glass, such as higher mechanical strength and deformation resistance. However, the light transmittance of pure sapphire is only about 86%, much lower than 92% for silica glass.
Anti-reflective coatings are used to increase light transmittance through an optically transparent substrate, such as glass or sapphire, by reducing the reflected fraction of incident light that passes through the substrate.
A single layer of transparent material having a refractive index between the refractive index of air, 1.0, and the refractive index of sapphire, 1.77, can provide an anti-reflective effect. However, the best effect will typically be achieved with a refractive index close to about 1.33.
In addition to a low refractive index, the anti-reflective coating must also have good film strength and a high hardness. However, with typical known polysiloxane coatings, it is difficult to obtain a relatively low refractive index for use with sapphire while also providing good film strength and high hardness.
Some typical anti-reflective coatings are provided using physical deposition of multiple layers of the coating to provide high uniformity. However, the process is typically complex, expensive, and provides poor film strength.
Improvements in the foregoing are desired.